The present invention relates to tissue engineering scaffolds.
Biomedical Engineering and Tooth Regeneration.
Tooth loss often results from a variety of oral diseases and physiological causes, including dental caries, periodontal disease, trauma, genetic disorders and aging (Amar 2003, Philstrom 2005, Kim 2006). Tooth loss can lead to physical and mental suffering that can lower an individual's self-esteem and quality of life (Amar 2003, Philstrom 2005, Kim 2006). Many forms of dental disease and some medical conditions like uncontrolled diabetes increase the risk of tooth loss. For the treatment of edentulism, the current options have been limited to the use of dental implants and/or conventional fixed or removable prostheses.
Recently, the emergence and development of biomedical engineering tools have led to a new scope of patient care in the field of medicine. For example, preliminary human clinical trials have reported of improved levels of bone formation in children with osteogenesis imperfecta, following systemic infusions of bone marrow stromal stem cells (BMSSC) or bone marrow cells (Horwitz 2001, Horwitz 2002). Recent advances in the fields of dental tissue engineering, materials science and stem cell biology suggest that tooth regeneration will be possible (Duailibi 2006). Additionally, the recent identification of different mesenchymal stem cells (MSCs) residing in dental or craniofacial tissues expands the scope of potential clinical benefits in helping to regenerate the dental tissues such as dentin, cementum and periodontal ligament (PDL) (Shi 2005). Dental tissue progenitor cells present in the pulp tissue of deciduous and adult teeth can be used to regenerate dentin and alveolar bone (Shi 2005; Zhang 2005). Additionally, cells isolated from both rat and pig tooth buds can be used to bioengineer anatomically correct tooth crowns but with limited predictability (Duailibi 2004, Honda 2005, Young 2002, Young 2005).
The tooth/periodontal complexes are often referred to as an individual organ. Although this organ is considered relatively small, its structural and developmental complexity is well recognized. The tooth structure consists of three calcified tissue types—enamel, dentin and cementum, and dental pulp. Dentin occupies the bulk of the tooth, while enamel and cementum cover the coronal and apical portions, respectively. The periodontium has a supportive role to the teeth and consists of cementum, periodontal ligaments, alveolar bone and gingiva. Periodontal ligaments are connective tissues that attach the cementum to the alveolar bone via the Sharpey's fibers. Periodontal ligaments enable sensory perception and cushion mechanical forces during mastication.
Despite the tooth's structural complexity, the advancement of biomedical engineering techniques has given rise to two currently employed approaches for tooth regeneration. The first is based on tissue engineering, aiming to regenerate teeth by seeding stem cells in scaffolding biomaterials (Young 2002, Duailibi 2004, Honda 2005). This technique has shown promising results in regeneration of the periodontium (Nakahara 2006). The second approach attempts to reproduce or mimic the developmental processes of embryonic tooth formation (Nakahara 2006). This approach uses embryonic tissues (dental epithelium and dental mesenchyme) harvested from a mouse fetus and requires an understanding of the principles that regulate early tooth development in the embryo (Ohazama 2004, Hu 2006, Nakao 2007). Following these approaches, in many studies, biologically engineered tooth germs are transplanted into the bodies of animal hosts, usually rodents, where there is sufficient blood flow to provide the necessary nutrients and oxygen to optimize tissue formation (Nakahara 2006).
Use of Stem Cells in Tissue Regeneration and Challenges Encountered.
Stem cells are quiescent cell populations present in normal tissue, which exhibit the distinct characteristic of asymmetric cell division, the formation two daughter cells—a new progenitor/stem cell, and another daughter cell capable of forming differentiated tissue (Hawkins 1998, Lin 1998). Dental mesenchymal progenitor cells have been identified and characterized in the dental pulp of both deciduous and adult human teeth (Gronthos 2000, Mooney 1996, Shi 2005). As previously mentioned, these postnatal epithelial and mesenchymal dental stem/progenitor cells present in immature tooth buds have demonstrated the ability to generate bioengineered and anatomically correct, but miniature-sized tooth crowns containing enamel, dentin, pulp, and alveolar bone (Shi 2005; Zhang 2005).
Periodontal ligament cells are known for their regenerative potential to give rise to the formation of lamina propria, cementum, bone, and periodontal ligament (Melcher 1985, McCulloh 1985). The capacity of periodontal ligament stem cells to form mineralized deposits in vitro has been demonstrated for a subpopulation of cells derived from primary explants of periodontal ligament (Arceo 1991, Cho 1992). It is believed that periodontal ligament stem cells require a suitable scaffold to induce the formation of bone, dentin and cementum in vivo (Gronthos 2000, Krebsbach 1998). When periodontal ligament stem cells were incorporated into a hydroxyapatite/tricalcium phosphate scaffolds and ectopically implanted in the subcutaneous regions of the mouse dorsum, a typical cementum/periodontal ligament-like structure formed (Seo 2004). Moreover, a type I collagen-positive periodontal ligament-like tissue within the transplants connecting with the newly formed cementum that is morphologically similar to Sharpey's fibers has been demonstrated (Seo 2004).
Recent advances in dental stem cell biotechnology and cell-mediated murine tooth regeneration have encouraged researchers to explore the potential for regenerating living teeth with appropriate functional properties (Duailibi 2004, Ohazama 2004, Shi 2005). Murine teeth can be regenerated using many different stem cells to collaboratively form dental structures in vivo (Duailibi 2004, Ohazama 2004, Young 2005). In addition, dentin/pulp tissue and cementum/periodontal complex have been regenerated by human dental pulp stem cells (DPSCs) and periodontal ligament stem cells (PDLSCs) respectively, when transplanted into immune-compromised mice (Gronthos 2000, Seo 2004). However, owing to the complexity of human tooth growth and development, the regeneration of a whole tooth structure including enamel, dentin/pulp complex, and periodontal tissues as a functional entity in humans is a challenge with the currently available regenerative biotechnologies (Sonomaya 2006).
The challenges with the use of stem cells in regeneration of dental tissues have been reported in previous studies (Duailibi 2004, Young 2002, Young 2005). It is acknowledged that, while formation of multiple miniature tooth crowns in the bioengineered tooth constructs is possible, real-size whole-tooth regeneration encounters a number of challenges. These challenges are attributed, again, to the complex nature of tooth development (Duailibi 2006, Tummers 2003).
Concepts of Cell Homing.
As put forward, conventional approaches of stem cell-seeding within a scaffold aim to mimic cellular structure and recreate a functional tissue equivalent in vitro or in vivo. The cells are derived from end organs or from more undifferentiated sources such as the bone marrow (Schantz 2007). These approaches are limited by issues such as donor site morbidity from harvesting of cells and tissue formation of heterogeneous quality at the site of implantation of the cell-scaffold construct (Schantz 2007). Hence, the concept of cell homing is recently attracting more attention. Cell homing aims to induce the homing of desired cells to cytokine-impregnated scaffolds at specific anatomical sites (Schantz 2007). This approach attempts in vivo tissue regeneration without cell-seeding. Therefore, cell homing could provide enhancements in cellular methodology for tissue engineering and a novel, minimally invasive option for tissue regeneration (Schantz 2007).
Based on the above discussion, further development of tooth scaffolds is needed. The present invention address that need.